Contribution of the paraventricular nucleus in autonomic adjustments to heat stress

Effects of physical training on hypothalamic neuronal activation and expressions of vasopressin and oxytocin in SHR after running until fatigue

To assess the influence of physical training on neuronal activation and hypothalamic expression of vasopressin and oxytocin in spontaneously hypertensive rats (SHR), untrained and trained normotensive rats and SHR were submitted to running until fatigue while internal body and tail temperatures were recorded. Hypothalamic c-Fos expression was evaluated in thermoregulatory centers such as the median preoptic nucleus (MnPO), medial preoptic nucleus (mPOA), paraventricular nucleus of the hypothalamus (PVN), and supraoptic nucleus (SON). The PVN and the SON were also investigated for vasopressin and oxytocin expressions. Although exercise training improved the workload performed by the animals, it was reduced in SHR and followed by increased internal body temperature due to tail vasodilation deficit. Physical training enhanced c-Fos expression in the MnPO, mPOA, and PVN of both strains, and these responses were attenuated in SHR. Vasopressin immunoreactivity in the PVN was also increased by physical training to a lesser extent in SHR. The already-reduced oxytocin expression in the PVN of SHR was increased in response to physical training. Within the SON, neuronal activation and the expressions of vasopressin and oxytocin were reduced by hypertension and unaffected by physical training. The data indicate that physical training counterbalances in part the negative effect of hypertension on hypothalamic neuronal activation elicited by exercise, as well as on the expression of vasopressin and oxytocin. These hypertension features seem to negatively influence the workload performed by SHR due to the hyperthermia derived from the inability of physical training to improve heat dissipation through skin vasodilation.

Temperature modulates PVN pre-sympathetic neurones via transient receptor potential ion channels

The paraventricular nucleus (PVN) of the hypothalamus plays a vital role in maintaining homeostasis and modulates cardiovascular function via autonomic pre-sympathetic neurones. We have previously shown that coupling between transient receptor potential cation channel subfamily V Member 4 (Trpv4) and small-conductance calcium-activated potassium channels (SK) in the PVN facilitate osmosensing, but since TRP channels are also thermosensitive, in this report we investigated the temperature sensitivity of these neurones. Methods: TRP channel mRNA was quantified from mouse PVN with RT-PCR and thermosensitivity of Trpv4-like PVN neuronal ion channels characterised with cell-attached patch-clamp electrophysiology. Following recovery of temperature-sensitive single-channel kinetic schema, we constructed a predictive stochastic mathematical model of these neurones and validated this with electrophysiological recordings of action current frequency. Results: 7 thermosensitive TRP channel genes were found in PVN punches. Trpv4 was the most abundant of these and was identified at the single channel level on PVN neurones. We investigated the thermosensitivity of these Trpv4-like channels; open probability (Po) markedly decreased when temperature was decreased, mediated by a decrease in mean open dwell times. Our neuronal model predicted that PVN spontaneous action current frequency (ACf) would increase as temperature is decreased and in our electrophysiological experiments, we found that ACf from PVN neurones was significantly higher at lower temperatures. The broad-spectrum channel blocker gadolinium (100 µM), was used to block the warm-activated, Ca2+-permeable Trpv4 channels. In the presence of gadolinium (100 µM), the temperature effect was largely retained. Using econazole (10 µM), a blocker of Trpm2, we found there were significant increases in overall ACf and the temperature effect was inhibited. Conclusion: Trpv4, the abundantly transcribed thermosensitive TRP channel gene in the PVN appears to contribute to intrinsic thermosensitive properties of PVN neurones. At physiological temperatures (37°C), we observed relatively low ACf primarily due to the activity of Trpm2 channels, whereas at room temperature, where most of the previous characterisation of PVN neuronal activity has been performed, ACf is much higher, and appears to be predominately due to reduced Trpv4 activity. This work gives insight into the fundamental mechanisms by which the body decodes temperature signals and maintains homeostasis.

Inhibition of nNOS in the paraventricular nucleus of hypothalamus decreases exercise-induced hyperthermia

The paraventricular nucleus of the hypothalamus (PVN) is an important site for autonomic control, which integrates thermoregulation centers and sympathetic outflow to thermoeffector organs. PVN neurons express the neuronal isoform of nitric oxide synthase (nNOS) whose expression is locally upregulated by physical exercise. Thus, the aim of the present study was to evaluate the role of nNOS in the PVN in the exercise-induced hyperthermia. Seven days after surgery, male Wistar rats received bilateral intra-PVN microinjections of the selective nNOS inhibitor Nw-Propyl-L-Arginine (NPLA) or vehicle (saline) and were submitted to an acute progressive exercise session on a treadmill until fatigue. Abdominal and tail skin temperature (T_abd and T_tail, respectively) were measured, and the threshold (Hthr; °C) and sensitivity (Hsen) for heat dissipation calculated. Performance variables were also collected. During the progressive exercise protocol, all animals displayed an increase in the T_abd. However, compared to vehicle group, the microinjection of NPLA in the PVN attenuated the exercise-induced hyperthermia. There was no difference in T_tail or Hthr between NPLA and control rats. In contrast, Hsen was increased in the NPLA group compared to vehicle. In addition, heat storage was lower in NPLA-treated animals. Despite the temperature differences, inhibition of nNOS in the PVN did not affect running performance on the treadmill. These results suggest that nitrergic signaling within the PVN, under nNOS activation, drives the increase of body temperature, being necessary for the proper thermal regulatory mechanisms during progressive exercise-induced hyperthermia.

β-Alanine Supplementation Attenuates the Neurophysiological Response in Animals Exposed to an Acute Heat Stress

The effect of 30 days of β-alanine supplementation on neurophysiological responses of animals exposed to an acute heat stress (HS) was examined. Animals were randomized to one of three groups; exposed to HS (120 min at 40-41 °C) and fed a normal diet (EXP; n = 12); EXP and supplemented with β-alanine (EXP + BA; n = 10); or not exposed (CTL; n = 10). Hippocampal (CA1, CA3 and DG) and hypothalamic (PVN) immunoreactive (ir) cell numbers of COX2, IBA-1, BDNF, NPY and HSP70 were analyzed. Three animals in EXP and one in EXP-BA did not survive the HS, however no significant difference (p = 0.146) was noted in survival rate in EXP + BA. The % change in rectal temperature was significantly lower (p = 0.04) in EXP + BA than EXP. Elevations (p's < 0.05) in COX-2, IBA-1 and HSP70 ir-cell numbers were noted in animals exposed to HS in all subregions. COX-2 ir-cell numbers were attenuated for EXP + BA in CA1 (p = 0.02) and PVN (p = 0.015) compared to EXP. No difference in COX-2 ir-cell numbers was noted between CTL and EXP + BA at CA1. BDNF-ir cell numbers in CA1, DG and PVN were reduced (p's < 0.05) during HS compared to CTL. No difference in BDNF-ir cell numbers was noted between EXP + BA and CTL in CA3 and PVN. NPY-ir density was reduced in exposed animals in all subregions, but NPY-ir density for EXP-BA was greater than EXP in CA3 (p < 0.001) and PVN (p = 0.04). β-Alanine supplementation attenuated the thermoregulatory and inflammatory responses and maintained neurotrophin and neuropeptide levels during acute HS. Further research is necessary to determine whether β-alanine supplementation can increase survival rate during a heat stress.

Systemic acyl-ghrelin increases tail skin temperature in rats without affecting their thermoregulatory behavior in a cold environment

Fasting increases ghrelin that is a peptide hormone with two circulating isoforms, acyl and des-acyl ghrelin. We reported that fasting or des-acyl ghrelin facilitates behavioral thermoregulation in the cold in rats assessed by tail-hiding behavior that was the indicator of rats' thermoregulatory behavior in the cold; however, the effect of acyl-ghrelin on the same process remains to be elucidated. We investigated the effect of acyl-ghrelin on thermoregulatory behavior in the cold in rats. The animals received an intraperitoneal saline or 24 μg acyl-ghrelin injection and were exposed to 27 °C or 15 °C for 2 h, while their body temperature, tail skin temperature, and tail-hiding behavior were constantly monitored. cFos immunoreactive (cFos-IR) cells in the median preoptic area, medial preoptic area, paraventricular nucleus (PVN), and arcuate nucleus were counted. Body temperature and the duration of thermoregulatory behavior did not show a significant difference between the acyl-ghrelin-treated and control groups at 15 °C; however, tail skin temperature in the acyl-ghrelin-treated group was higher than that in the control group. The number of cFos-IR cells in the PVN was greater in the control group than that in the acyl-ghrelin-treated group at 27 °C. These results indicate that acyl-ghrelin did not affect behavioral thermoregulation but might affect tail skin temperature in rats in the cold.

Impaired thermoregulation in spontaneously hypertensive rats during physical exercise is related to reduced hypothalamic neuronal activation

This study aimed to evaluate the physical exercise-induced neuronal activation in brain nuclei controlling thermoregulatory responses in hypertensive and normotensive rats. Sixteen-week-old male normotensive Wistar rats (NWRs) and spontaneously hypertensive rats (SHRs) were implanted with an abdominal temperature sensor. After recovery, the animals were subjected to a constant-speed treadmill running (at 60% of the maximum aerobic speed) for 30 min at 25 °C. Core (T_core) and tail-skin (T_skin) temperatures were measured every minute during exercise. Ninety minutes after the exercise, the rats were euthanized, and their brains were collected to determine the c-Fos protein expression in the following areas that modulate thermoregulatory responses: medial preoptic area (mPOA), paraventricular hypothalamic nucleus (PVN), and supraoptic nucleus (SON). During treadmill running, the SHR group exhibited a greater increase in T_core and an augmented threshold for cutaneous heat loss relative to the NWR group. In addition, the SHRs showed reduced neuronal activation in the mPOA (< 49.7%) and PVN (< 44.2%), but not in the SON. The lower exercise-induced activation in the mPOA and PVN in hypertensive rats was strongly related to the delayed onset of cutaneous heat loss. We conclude that the enhanced exercise-induced hyperthermia in hypertensive rats can be partially explained by a delayed cutaneous heat loss, which is, in turn, associated with reduced activation of brain areas modulating thermoregulatory responses.

Central losartan administration increases cardiac workload during aerobic exercise

To assess the effects of central administration of losartan, an antagonist of angiotensin II AT₁ receptors, on cardiovascular function during aerobic exercise, heart rate, systolic and diastolic arterial pressures and rate pressure product of Wistar rats were measured as cardiac workload indexes. The animals ran on a treadmill until fatigue after an intracerebroventricular injection of losartan or saline. Pulsatile arterial pressure was recorded by a catheter implanted into the ascending aorta, from which were derived cardiovascular parameters to estimate the cardiac workload. Total exercise time and exercise workload were determined as performance indexes. The rats showed a more intense increase in heart rate after 8 min of exercise and sustained until fatigue (P < .05). Furthermore, the rats injected with losartan had a higher increase of both systolic and diastolic arterial pressures as well as rate pressure product from approximately 6 min of exercise until fatigued (P < .05). In addition, a 22% reduction in exercise time was found in losartan-rats (P < .01). This ergolytic effect induced by losartan was strongly inversely correlated with rate-pressure product during aerobic exercise (r = 0.78, P ≤ .01). The data shows that central administration of losartan augments the cardiac workload during aerobic exercise, which courses in parallel with the reduced exercise performance.

Ion Channels in the Paraventricular Hypothalamic Nucleus (PVN); Emerging Diversity and Functional Roles

The paraventricular nucleus of the hypothalamus (PVN) is critical for the regulation of homeostatic function. Although also important for endocrine regulation, it has been referred to as the "autonomic master controller." The emerging consensus is that the PVN is a multifunctional nucleus, with autonomic roles including (but not limited to) coordination of cardiovascular, thermoregulatory, metabolic, circadian and stress responses. However, the cellular mechanisms underlying these multifunctional roles remain poorly understood. Neurones from the PVN project to and can alter the function of sympathetic control regions in the medulla and spinal cord. Dysfunction of sympathetic pre-autonomic neurones (typically hyperactivity) is linked to several diseases including hypertension and heart failure and targeting this region with specific pharmacological or biological agents is a promising area of medical research. However, to facilitate future medical exploitation of the PVN, more detailed models of its neuronal control are required; populated by a greater compliment of constituent ion channels. Whilst the cytoarchitecture, projections and neurotransmitters present in the PVN are reasonably well documented, there have been fewer studies on the expression and interplay of ion channels. In this review we bring together an up to date analysis of PVN ion channel studies and discuss how these channels may interact to control, in particular, the activity of the sympathetic system.

Central neural control of thermoregulation and brown adipose tissue

Central neural circuits orchestrate the homeostatic repertoire that maintains body temperature during environmental temperature challenges and alters body temperature during the inflammatory response. This review summarizes the experimental underpinnings of our current model of the CNS pathways controlling the principal thermoeffectors for body temperature regulation: cutaneous vasoconstriction controlling heat loss, and shivering and brown adipose tissue for thermogenesis. The activation of these effectors is regulated by parallel but distinct, effector-specific, core efferent pathways within the CNS that share a common peripheral thermal sensory input. Via the lateral parabrachial nucleus, skin thermal afferent input reaches the hypothalamic preoptic area to inhibit warm-sensitive, inhibitory output neurons which control heat production by inhibiting thermogenesis-promoting neurons in the dorsomedial hypothalamus that project to thermogenesis-controlling premotor neurons in the rostral ventromedial medulla, including the raphe pallidus, that descend to provide the excitation of spinal circuits necessary to drive thermogenic thermal effectors. A distinct population of warm-sensitive preoptic neurons controls heat loss through an inhibitory input to raphe pallidus sympathetic premotor neurons controlling cutaneous vasoconstriction. The model proposed for central thermoregulatory control provides a useful platform for further understanding of the functional organization of central thermoregulation and elucidating the hypothalamic circuitry and neurotransmitters involved in body temperature regulation.

Temperature Control of Hypertensive Rats during Moderate Exercise in Warm Environment

The control of body temperature in Spontaneously Hypertensive Rat (SHR) subjected to exercise in warm environment was investigated. Male SHR and Wistar rats were submitted to moderate exercise in temperate (25°C) and warm (32°C) environments while body and tail skin temperatures, as well as oxygen consumption, were registered. Total time of exercise, workload performed, mechanical efficiency and heat storage were determined. SHR had increased heat production and body temperature at the end of exercise, reduced mechanical efficiency and increased heat storage (p < 0.05). Furthermore, these rats also showed a more intense and faster increase in body temperature during moderate exercise in the warm environment (p < 0.05). The lower mechanical efficiency seen in SHR was closely correlated with their higher body temperature at the point of fatigue in warm environment (p < 0.05). Our results indicate that SHR exhibit significant differences in body temperature control during moderate exercise in warm environment characterized by increased heat production and heat storage during moderate exercise in warm environment. The combination of these responses result in aggravated hyperthermia linked with lower mechanical efficiency. Key PointsThe practice of physical exercise in warm environment has gained importance in recent decades mainly because of the progressive increases in environmental temperature;To the best of our knowledge, these is the first study to analyze body temperature control of SHR during moderate exercise in warm environment;SHR showed increased heat production and heat storage that resulted in higher body temperature at the end of exercise;SHR showed reduced mechanical efficiency;These results demonstrate that when exercising in a warm environment the hypertensive rat exhibit differences in temperature control.

Construction and validation of lentiviral vector carrying rat neuronal nitric oxide synthase in vitro and in vivo

In the present study, we developed a lentiviral vector with human cytomegalovirus promoter permitting high-level of nNOS expression. Neuronal cell line NG108 was used as an in vitro model to check the validity of gene transfer. The cells were infected with lenti-EGFP or lenti-nNOS particles for 24h. Lenti-nNOS infection in the NG108 cells induced dose dependent increase in mRNA and protein for nNOS; with a dose of 2.5 × 10⁴ pfu/ml, nNOS mRNA expression increased by 40-fold while protein expression was increased by 2.5-fold compared to lenti-EGFP. Moreover, lenti-nNOS infection caused a greater increase in nNOS immunoreactivity in NG108 cells compared to lenti-EGFP as shown by immonocytochemistry. nNOS expression showed time dependent increases with lenti-nNOS infection with maximum up-regulation observed after two weeks of infection. Moreover, in vivo, unilateral injection of lenti-nNOS into the paraventricular nucleus (PVN) of rats induced a 27-fold increase of nNOS protein level in the injected side compared to non-injected side and this escalation was sustained up to three weeks. Overall, lenti-EGFP injection in the PVN did not show any significant change in nNOS expression. Furthermore, NADPH-diaphorase staining of nNOS in the PVN infected with lenti-nNOS induced a visible increase in nNOS expression compared with contralateral non-injected side up to three weeks. These results indicate that this approach of lentiviral mediated gene transfer of nNOS may provide a new means to up-regulate the nNOS expression for longer periods of time compared to adenoviral transfection and can be used as a research tool and potentially a therapy for chronic diseases involving impaired nNOS expression.